For once, United Airlines offers a promotion on MilesPlus miles, and the calculations may actually add up if you're a member.It's a once in a lifetime offer for United's MilesPlus program, barring any changes to their policy, where the math might make sense. It's more significant than it may seem purchasing miles directly from an airline is typically one of the worst travel offers ever. When such sales occur, however, they can turn that equation upside down for the right traveler who has the right redemption in mind.

The upfront clue: Not everybody is getting the same break. While one person may be able to get a 100% bonus when they log in, another may only receive a smaller bonus. The best way to determine what you're dealing with is to log in to your own MileagePlus account and find out.

What's On The Table?

The best deals around right now are 2 forms of a 100% bonus on miles purchased or a 50% discount on purchase price.There are some members getting less of this same promotion, such as bonuses that are as low as 80% or discounts that fall around 40% and there are plenty of accounts getting something in the middle. You need to do a minimum of 20,000 miles in one purchase to secure the highest rate, before bonus miles kick in.

United prices are normally $0.035 per mile purchased plus 7.5% federal excise tax applied as a percentage. The maximum purchase amount is 50,000 miles and here's how much that will cost you after the deal expires:

• At a 40% discount (or 80% bonus), you'd pay $1,128.75 total, which works out to about 2.26 cents per mile.
• At a 45% discount (or 90% bonus), the total drops to $1,034.69, or roughly 2.07 cents per mile.
• If you are at the top tier 50% discount, or 100% bonus, then you are looking at $940.63, which is only $0.0188 per mile.

How Many Miles Do You Really Get?

United places a limit on the number of miles that can be bought, and it's not as straightforward as an annual limit. Up to 200,000 MileagePlus miles can be earned every calendar year, but there is a rolling restriction that allows for purchases of a maximum of 50,000 miles in a 90 day period. Even with a massive 100% bonus, neither of these caps will be able to be bypassed after three months: 50,000 miles after each bonus period. After making the purchase, allow for up to 48 hours to receive the miles.

What Credit Card to Use to Purchase Miles?

One item that often confuses people is that all the miles purchased by United go through a third party processor, points.com, and it is not recognized as an airline purchase on the credit card. So if you had hoped to earn travel points with the purchase of Miles, you won't find it to be true here.

Rather, it's best to choose a card that will either help you reach a minimum spending goal to earn a new signup bonus or will offer a high flat rate on regular purchases. A handful of solid cards to consider for this type of nonbonused spending are the Citi Double Cash Card, The Chase Freedom Unlimited and The Blue Business Plus Credit Card from American Express.

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Why is it that United Miles is not worthwhile?

It's more about the percentage of the discount than the actual question. To me, one of the only redeemed ways to spend United miles is to book United Polaris business class. More and more, United has been closing the doors to its long haul business class award space, favoring members with a co-brand credit card or elite status. United's route network is vast, so it's not the flashiest business class product flying these days but that's a major advantage when travelling for a specific destination.

Other than that use case, I don't feel quite as enthusiastic. United's redemption rates on other airline partners in the Star Alliance are often below that of other programs in the alliance. Air Canada Aeroplan and Avianca LifeMiles are more likely to provide a better value for partner bookings, and both mileage programs often have their own sales of miles at a lower cost than United's. I must mention the stopover allowance which is generous on Aeroplan, but not United, and makes it a no brainer if you're making a tour with several stops.

Where can you redeem United miles? Well, United is a member of Star Alliance, which means your miles will be accepted on any other Star Alliance airline, and some other partners, such as Aer Lingus, Azul and Discover Airlines.

What Value do United Miles Really Have?

While everyone has their own idea of the value of airline miles, I believe that the United MileagePlus miles (a pretty conservative estimate) are worth about 1.3 cents each. There is a lot more to be gained from them with some clever redemptions, especially for premium cabins fares on international long haul flights.

So far at least the good news is that MileagePlus miles never expire.Your account remains active regardless of how many times or not you use United's services, and there is no minimum activity requirement to maintain the account.

MileagePlus will also be a 1:1 transfer partner with Chase Ultimate Rewards, meaning that you will be able to transfer your Chase points to the Miles Plus program without paying anything extra.

The Bottom Line

Right now, it's a great mileage sale for United, the highest being available on the buy as little as 1.88 per mile deal, which is now 50 percent off or 100 percent match. However, not all members will receive that offer in their account as the targeting differs from member to member and some will only be eligible to earn a lower bonus or discount.

The offer is good to consider if you have a particular redemption in mind, particularly one such as a United Polaris business class seat, which can otherwise be a difficult one to get. No matter how attractive the discount may appear on the surface, it is a poor choice to speculatively purchase miles without a clear strategy of how you will use them.